Fillers with Controlled Interfaces

By choosing the appropriate chemical structures chains that span filler particles in a PDMS-based composite can be designed so that they are durable, irreversibly breakable, or breakable reversibly. 

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Interfacial structure is known to be different from bulk structure, and in polymers filled with nanofillers possessing extremely high specific surface areas, most of the polymers is present near the interface, in spite of the small weight fraction of filler. This is one of the reasons why the nature of the reinforcement is different in nanocomposites and is manifested even at very low filler loadings (<10 wt%). Crucial parameters in determining the effect of fillers on the properties of composites are filler size, shape, aspect ratio, and filler-matrix interactions [2-5]. In the case of nanocomposites, the properties of the material are more tied to the interface. Thus, the control and manipulation of microstructural evolution is essential for the growth of a strong polymer-filler interface in such nanocomposites. 

Fumed silica is widely used for the reinforcement of polydimethylsiloxane (PDMS) elastomers. The intermolecular interaction of the filler surface with the PDMS matrix controls this process [1, 2] so that understanding which factors influence the interaction at the filler/PDMS interface has become a crucial point for further development of the technology. Among the factors of interest there are the... 

It is generally acknowledged that two distinct factors contribute to the final localization of the fillers thermodynamic effects, based on the affinity that the fillers might have with the polymers and kinetic effects that include the mode of preparation of the composites and the ability of the fillers to migrate to a different location, i.e., from one phase to the other or from one phase to the interface. These factors are critical in the prediction and control of filler localization and are described in detail below. 

Reinforcement depends on two features the number of interactions at the interface between polymer and filler (which is mainly controlled by the low primary particle size in conjunction with the surface activity) and the hydrodynamic effects of particle aggregation and agglomeration (which are linked with shear modulus and hysteresis during dynamic or static deformation). 

A particularly important area of fundamental research and technology hinges upon the control of the adhesion between the polymer matrix and the filler. Completely unbonded particles appear to have the same effect as voids in many types of behaviour. The quality of the interface and its response during deformation and as well as its sensitivity to environmental changes are of major interest. Nelson and Hancock, and Kendall have examined the consequences of interfacial slip in composites and underlined its importance. In general terms this aspect of composite technology has very much in common with the design of structural adhesive Joints and hence the appropriate references of the previous section are of value. 

Formulating BMIs as matrices for composites and hybrid materials is another effective approach. The use of micro- and nanometer scale fillers allowed materials with new or improved properties. Studies of interactions at the interface among nanometric particles and a multicomponent polymer matrix have indicated that the interface itself can be equally important as the individual components regarding the overall effects because not all the principles from macro- and microscale can be used to explain the properties and behavior of nanocomposites. Combining these methods provides the ability to tailor and control the overall composition of these new materials, their structure (nanostructure, as well as supramolecular architecture), and also allows tunable properties by tunable structure-property relationship. 

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